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Plated microvias are widely used within todays PCB industry as a means of achieving the high-density designs that are required in modern mobile devices, however, there has been growing concern regarding their long term reliability performance when stacked directly on top of each other. Blind microvias (BMV) have a potentially complex metallurgical structure, with several interfaces located around the target pad - electroless Copper - electrolytic Copper joint. While field experience as shown that there are typically two major types of crystal structures formed across the BMV base, there has been little reported work investigating how or why such structures develop. In this paper, we review these two commonly observed microstructures within filled BMVs and offer proposals on how such structures are created. We subsequently describe a novel means to indicate if the microstructure of a BMV is likely to have a tendency for an early onset of failure.